JP5398297B2 - Method for producing porous carbon electrode substrate - Google Patents

Description

  The present invention relates to a porous carbon electrode substrate and a method for producing the same.

  A porous carbon electrode base material based on carbon fiber is used for a gas diffusion layer for a polymer electrolyte fuel cell. The gas diffusion layer needs to transport a reaction gas such as hydrogen and oxygen used for the electrode reaction to the catalyst layer where the electrode reaction is efficiently performed. In applications that require high power density, such as automobile applications, the fuel cell is operated in a region where the current density is high, so the amount of gas required per unit area and the amount of water generated increase. Therefore, in such a case, it is important to efficiently supply the gas necessary for the reaction and efficiently discharge the water produced by the reaction, and it is important to control the gas flow and the water flow.

  With respect to the above problem, Patent Document 1 describes a porous carbon electrode base material in which a gas easily spreads in a plane by mixing fibers that are burned off during firing. However, since the porous carbon electrode base material manufactured by such a method has a gas permeability in the plane direction that is too high with respect to the penetration direction, the discharge efficiency of the generated water is lowered. Moreover, since the fiber with a large diameter is mixed, unevenness becomes large. Therefore, there is a concern that other members may be damaged when the cell stack is assembled, and further improvement is desired.

  Patent Document 2 proposes a method in which a layer having a low bulk density is placed between two dense layers. Although the bulk density of the surface layer is increased in order to suppress the compressive residual strain, this reduces the overall gas permeability, which is insufficient from the viewpoint of gas permeability and drainage.

  Patent Document 3 discloses a method of increasing the electrical conductivity in the in-plane direction of the gas diffusion electrode base material between the gas flow channel grooves by orienting the carbon fibers. By such a method, the gas permeability in a plane in one direction is increased, but it cannot be said that the gas permeability in the plane direction is sufficiently large with respect to the penetration direction, and further improvement is desired.

JP 2006-190518 A JP 2007-176750 A JP 2006-222024 A

The present invention provides a method for producing a porous carbon electrode substrate that can efficiently distribute a reaction gas to a fuel cell electrode reaction part when used in a gas diffusion layer for a fuel cell.

The method for producing a porous carbon electrode substrate according to the present invention comprises:
A method for producing a porous carbon electrode substrate in which two or more carbon short fiber papers are laminated via a phenolic resin carbide, and the degree of fiber orientation of the carbon short fibers is 2 to 5,
(1) A step of making a paper material containing short carbon fibers and a binder,
(2) The paper stock after paper making is continuously sandwiched between felts for paper making and pressed at a pressure of 0 to 0.05 MPa to make the moisture content 80 to 85%, and then dried to produce carbon short fiber paper. Obtaining a step,
(3) impregnating and laminating two or more carbon short fiber papers with a phenolic resin;
(4) A step of carbonizing the laminated short carbon fiber paper while pressing the phenol resin by pressing at a pressure of 5 to 18 MPa while heating,
It is the method which has these.

  According to the method of the present invention, it is possible to manufacture a porous carbon electrode base material in which the gas permeability in the penetration direction and the plane direction is optimized, and when used in a gas diffusion layer for a fuel cell, the fuel cell The reaction gas can be efficiently distributed to the electrode reaction part.

It is the schematic which shows the press of the stock after papermaking in this invention, and a drying process. It is the figure which showed an example of the gas permeability measuring apparatus which measures the gas permeability of a surface direction. It is the figure which showed the mode of the pressure distribution observed in the measuring method of the maximum surface pressure. It is the figure which showed the cross-sectional photograph by the fluorescence microscope of the resin hardening carbon short fiber paper in Example 1. FIG.

[Method for producing porous carbon electrode substrate]
The method for producing a porous carbon electrode substrate according to the present invention comprises:
A method for producing a porous carbon electrode substrate in which two or more carbon short fiber papers are laminated via a phenolic resin carbide, and the degree of fiber orientation of the carbon short fibers is 2 to 5,
(1) A step of making a paper material containing short carbon fibers and a binder,
(2) The paper stock after paper making is continuously sandwiched between felts for paper making and pressed at a pressure of 0 to 0.05 MPa to make the moisture content 80 to 85%, and then dried to produce carbon short fiber paper. Obtaining a step,
(3) pre-Symbol phenol resin impregnated into short carbon fiber paper, laminating two or more short carbon fiber paper impregnated with phenolic resin,
(4) A step of carbonizing the laminated short carbon fiber paper while pressing the phenol resin by pressing at a pressure of 5 to 18 MPa while heating,
It is the method which has these.

[Process (1): Paper making process]
In the production method according to the present invention, first, a paper material containing short carbon fibers and a binder is made.

<Short carbon fiber>
Examples of the carbon fiber that is a raw material of the short carbon fiber used in the present invention include polyacrylonitrile-based carbon fiber, pitch-based carbon fiber, and rayon-based carbon fiber, and polyacrylonitrile-based carbon fiber is preferable. In particular, it is more preferable that the carbon fiber to be used is only polyacrylonitrile-based carbon fiber because the mechanical strength of the porous carbon electrode substrate can be made relatively high.

  The diameter of the short carbon fiber is preferably 3 to 9 μm from the viewpoint of production cost and dispersibility of the short carbon fiber. Moreover, it is more preferable from the surface of the smoothness of a porous carbon electrode base material that it is 4-8 micrometers. The fiber length of the short carbon fibers is preferably 2 to 12 mm from the viewpoint of dispersibility.

<Binder>
In the production method of the present invention, any binder can be used as long as it is an organic polymer compound that can function as a glue that holds each component in carbon fiber paper by paper making with short carbon fibers and can form a stock. However, it is particularly preferable to use polyvinyl alcohol. Polyvinyl alcohol is preferable as a binder because it has excellent binding power in the paper making process, and the short carbon fibers do not fall off. In the present invention, it is also possible to use a hydrophilic resin such as polyvinyl alcohol as a fiber. A hydrophilic resin such as polyvinyl alcohol swells in hot water and binds carbon fibers after drying. The higher the moisture content, the larger the swelling. In the method of the present invention, moisture collects on the lower surface of the paper making, so that a hydrophilic resin such as polyvinyl alcohol swells on the lower surface of the paper making and strongly binds the carbon fibers.

<Paper making method>
In the present invention, the papermaking method is not particularly limited, but papermaking can be performed by, for example, a wet papermaking method. Specifically, after a paper stock containing carbon short fibers and a binder is uniformly dispersed in water, paper can be continuously produced by a wet continuous paper machine. The mass ratio of the short carbon fibers to be dispersed in water and the binder is preferably short carbon fibers / binder = 70/30 to 95/5. The mass ratio of the stock and water is preferably stock / water = 1/50 to 1/1000. The moisture content of the stock after paper making is preferably 85 to 90%. The moisture content of the stock after paper making in this paper making process can be adjusted by adjusting the strength of dehydration when the felt is spread or the dispersion liquid is sprinkled.

[Step (2): Pressing with paper felt, drying step]
Next, the paper stock after paper making is sandwiched between felts for paper making and pressed at a pressure of 0 to 0.05 MPa to make the moisture content 80 to 85%, and then dried to obtain carbon short fiber paper.

<Felt for papermaking>
The papermaking felt used in the production method of the present invention preferably has a function of transferring a paper stock after papermaking containing a lot of water, a function of squeezing moisture when pressed, and a function of smoothing the surface. For example, a papermaking felt manufactured by Nippon Felt Co., Ltd. can be mentioned.

<Pressing pressure>
In the production method of the present invention, the paper material after paper making is sandwiched between felts for paper making and pressed at a pressure of 0 to 0.05 MPa. Since the paper stock after paper making obtained by the wet paper making method contains a lot of moisture, it is sandwiched and pressed between papermaking felts before drying to adjust the moisture content.

  For example, as shown in FIG. 1, the paper stock 1 after papermaking is continuously conveyed by a papermaking felt 2 and pressed by the two rollers through the papermaking felt 2.

  The pressing pressure in the present invention is 0 to 0.05 MPa, preferably 0 to 0.02 MPa. When the pressure exceeds 0.05 MPa, not only the moisture content is smaller than the range specified in the present invention, but also the moisture contained in the felt surface for papermaking spreads throughout the stock, the distribution of binder, and finally Since the distribution of the phenol resin carbide becomes uniform, the gas permeability in the surface direction cannot be increased. In order to produce a porous carbon electrode base material with an optimized gas permeability, it is essential that moisture is collected on the papermaking felt surface. If the moisture content of the stock after paper making is within the range of the present invention, it is not necessary to press.

  In the case of general paper, the pulp is sufficiently intertwined, so there is no problem with pressing between papermaking felts, but in the case of carbon fiber paper, the paper is peeled off or the papermaking felt is used. Since it may stick, polyethylene pulp or vinylon fiber may be mixed with the paper stock. On the other hand, if the moisture content is set high in advance, when the binder is dried, it adheres more strongly to the carbon fiber, so that it is not necessary to mix polyethylene pulp or vinylon fiber.

<Moisture content>
In the production method of the present invention, it is essential that the moisture content contained in the pressed paper is 80 to 85%. When the moisture content is less than 80%, the strength of the short carbon fiber paper is low because the effect of the binder when dried is low, and a porous carbon electrode substrate having a preferable gas permeability cannot be obtained. When the moisture content is larger than 85%, water is not sufficiently squeezed, and thus uneven drying tends to occur.

  The moisture content can be adjusted by controlling the pressing pressure as described above. Moreover, when not pressing (pressing pressure: 0 MPa), the moisture content is adjusted to 80 to 85% by water absorption by the papermaking felt.

<Method for measuring moisture content>
The moisture content can be measured by sampling the paper stock after paper making and measuring the mass before and after drying and by monitoring a non-contact type moisture meter in the line. In the present invention, the former before and after drying. This is done by measuring the mass. The moisture content is calculated by the following formula.
Moisture content (%) = (1-mass after drying / mass before drying) × 100.

<Dry>
After the pressing, the post-paper stock is dried. In order to connect the short carbon fiber and the short carbon fiber with a binder, heat is applied in the state where moisture remains, and the carbon short fiber is dried. When dried without applying heat, the binder does not swell, so the strength of the short carbon fiber paper becomes weak, which is not preferable.

  For example, as shown in FIG. 1, the drying can evaporate moisture by continuously pressing the stock 1 after paper making and then contacting the hot roll 3. The temperature of the hot roll 3 is preferably 100 to 140 ° C. from the viewpoint of maintaining strength and elongation for stable papermaking. Carbon short fiber paper can be obtained by the drying step.

[Process (3): Impregnation with phenolic resin, lamination process]
Next, two or more carbon short fiber papers are impregnated with a phenol resin and laminated.

<Phenolic resin>
As a phenol resin used by this invention, the resol type phenol resin obtained by reaction of phenols and aldehydes in presence of an alkali catalyst can be mentioned.

  The resol type phenol resin can also be dissolved and mixed with a novolac type phenol resin which is produced by the reaction of phenols and aldehydes in the presence of an acidic catalyst by a known method and exhibits solid heat-fusibility. In this case, a self-crosslinking type containing a curing agent such as hexamethylenediamine is preferred.

  Examples of the phenols include phenol, resorcin, cresol, xylenol, and the like. Examples of the aldehydes include formaldehyde (formalin), paraformaldehyde, furfural and the like. Moreover, these can be used as a mixture. Commercially available products can also be used as the phenol resin, and for example, “Phenolite J-325” (trade name, manufactured by DIC Corporation) and the like can be used.

<Impregnation method of phenol resin>
The method for impregnating the carbon short fiber paper with the phenol resin is not particularly limited as long as the carbon short fiber paper can be impregnated with the phenol resin. However, a method of uniformly coating a phenolic resin on the surface of carbon short fiber paper using a coater (for example, kiss coating method), a dip-nip method using a drawing device, or a resin film containing carbon short fiber paper and phenolic resin The method of transferring the phenol resin to the carbon short fiber paper in a superimposed manner is preferable in that it can be continuously performed and productivity and a long product can be manufactured.

<Phenolic resin amount>
The resin amount of the phenol resin impregnated in the carbon short fiber paper is preferably 70 to 150 parts by mass with respect to 100 parts by mass of the carbon short fiber contained in the carbon short fiber paper. In order to produce a porous carbon electrode substrate having a high gas permeability, it is necessary to impregnate the phenol resin so that the ratio of the phenol resin carbide in the porous carbon electrode substrate is 25 to 40% by mass. Therefore, it is preferable to impregnate 70 to 150 parts by mass of phenol resin.

<Lamination of carbon short fiber paper>
Since the carbon short fiber paper of the present invention is controlled so as not to be pressed or to be as small as possible, the adhesive force of the binder on one side is strong and the other is weak. In such a case, if the carbon short fiber paper is baked without being laminated, a difference occurs in the shrinkage rate on both sides, and the sheet may be warped. For this reason, it is essential to laminate two or more carbon short fiber papers so that the shrinkage rate is symmetric.

  Moreover, in this invention, it is preferable to laminate | stack carbon short fiber paper so that the papermaking lower surface side may become the surface. In the short carbon fiber paper of the present invention, the binder is greatly swollen and firmly held on the lower surface of the paper, so that the phenol resin carbide is collected on the lower surface of the paper as the final product. By making the papermaking lower surface side the surface, the surface layer has a large amount of phenol resin carbide, and the inner layer has a structure with less phenol resin carbide. In order to optimize the gas permeability, that is, to increase the gas permeability in the in-plane direction, it is preferable that the gas permeability inside is better than the surface layer of the porous carbon electrode substrate.

[Process (4): Phenolic resin curing and carbonization process]
Two or more carbon short fiber papers impregnated with the phenol resin are pressed at a pressure of 5 to 18 MPa while being heated to cure the phenol resin, and then carbonized.

<Phenolic resin curing method>
As a method of curing the phenolic resin contained in the carbon short fiber paper laminated two or more sheets, a method of hot pressing with a smooth rigid plate from both the upper and lower surfaces and a method of pressing using a continuous belt press device are used. Can do. Thereby, not only the phenol resin can be cured, but also the carbon short fiber paper surface can be made smooth. In particular, a method of pressing using a continuous belt press apparatus is preferable from the viewpoint of producing a long porous carbon electrode substrate.

  There are two methods of pressing in a continuous belt press device: a method of applying pressure to the belt with a roll press using a linear pressure and a method of pressing with a surface pressure using a hydraulic head press. The latter is a smoother porous carbon electrode substrate. This is preferable in that a material is obtained. In order to effectively smooth the surface, a method of pressing at a temperature at which the phenol resin is most softened and then fixing the phenol resin by heating or cooling is more preferable.

<Pressing pressure>
The pressure for pressing the carbon short fiber paper impregnated with two or more phenolic resins needs to be 5 to 18 MPa. When the pressing pressure is lower than 5 MPa, the two sheets are not sufficiently bonded and peeled off. On the other hand, when the pressing pressure is higher than 18 MPa, the bulk density increases and the gas permeability decreases in both the surface direction and the penetration direction, which is not preferable. Preferably, the press pressure is 7 to 15 MPa.

  In addition, when curing the phenolic resin impregnated in the carbon short fiber paper with a continuous belt press device sandwiched between rigid plates, apply a release agent in advance so that the phenolic resin does not adhere to the rigid plate or belt, Alternatively, it is preferable to perform press with a release paper sandwiched between carbon short fiber paper and a rigid plate or belt because the surface can be further smoothed.

<Carbonization method of phenol resin>
The carbonization of the phenol resin after curing is preferably performed by continuous firing over the entire length of the short carbon fiber paper in a temperature range of 1000 to 3000 ° C. in an inert treatment atmosphere. Carbonization can be performed in an inert gas to increase the conductivity of the porous carbon electrode substrate. Moreover, it is preferable to carry out continuously over the entire length of the carbon short fiber paper. If the porous carbon electrode base material is long, not only the productivity of the porous carbon electrode base material is improved, but also the Membrane Electrode Assembly (MEA) production in the subsequent process can be performed continuously, and the fuel cell This can greatly contribute to cost reduction. In the carbonization of the present invention, a pretreatment in an inert atmosphere of about 300 to 800 ° C. may be performed before the carbonization treatment for firing in a temperature range of 1000 to 3000 ° C. in an inert atmosphere. .

  The desired porous carbon electrode substrate can be obtained by the above steps.

<Fiber orientation>
The porous carbon electrode substrate produced by the method of the present invention has a fiber orientation degree of short carbon fibers of 2 to 5, preferably 2.5 to 4. The fiber orientation degree is calculated from the bending strength ratio (MD / CD) in the longitudinal direction (MD direction, penetration direction) and lateral direction (CD direction, plane direction) of the porous carbon electrode substrate.

  When the fiber orientation degree is less than 2, the reaction gas flowing from the separator gas flow path does not spread sufficiently to the reaction part of the catalyst layer, which is not preferable. On the other hand, when the fiber orientation degree is larger than 5, it is not preferable because the thickness becomes thin in the step of curing the phenol resin, and it is difficult to obtain a preferable bulk density described later.

<Method for orienting short carbon fibers>
Examples of the method for orienting the short carbon fibers include a method of dehydrating the supplied paper stock in the first pre-dehydration zone on the wire side in the paper making step of the step (1). In general, when a carbon short fiber dispersion is sprinkled on a wire at a high flow rate and water is rapidly dehydrated, the carbon short fibers are oriented. Thereby, the fiber orientation degree can be controlled within the above range.

<Measurement method of fiber orientation degree>
The fiber orientation degree can be determined from the bending strength of the porous carbon electrode substrate. For the measurement of the bending strength, first, 10 sheets are cut into a size of 80 mm × 10 mm so that the MD direction or CD direction of the porous carbon electrode substrate is the long side of the test piece, respectively. Using a bending strength test device (trade name: “SV-200”, manufactured by Imada Seisakusho Co., Ltd.), the distance between the fulcrums is 20 mm, and a load is applied to the test piece at a strain rate of 10 mm / min. The breaking load of the pressure wedge when the test piece breaks from the measured points is measured for 10 test pieces and is obtained from the following equation.

P: Breaking load (N)
L: Distance between fulcrums (mm)
W: Specimen width (mm)
h: Height of the test piece (mm).

[Porous carbon electrode substrate]
The porous carbon electrode substrate according to the present invention is a porous carbon electrode substrate produced by the method according to the present invention, and satisfies the following (A) and (B).
(A) face the direction of the gas permeability is 0.05~0.10ml / min / Pa (Measurement conditions: Gas flow rate 200 ml / min gas permeation area 0.785 cm ²⁾
(B) (Gas permeability in the penetration direction) / (Gas permeability in the surface direction) is 100 to 200.

The porous carbon electrode substrate according to the present invention preferably satisfies the following conditions (C) and (D) in addition to the above (A) and (B).
(C) Bulk pressure is 0.23 to 0.30 g / m ² (D) When a pressure of 1.0 MPa is applied, the maximum surface pressure is 1.0 to 2.0 MPa.

<Gas permeability in the surface direction>
The porous carbon electrode substrate according to the present invention has a gas permeability in the plane direction of 0.05 to 0.10 ml / min / Pa (measuring condition: gas flow rate 200 ml / min gas permeation area 0.785 cm ² ). Permeating the gas in the surface direction is an important function of the porous carbon electrode base material, and when this value is within the appropriate range, the reaction gas can be transported to the reaction part and the generated water discharged efficiently. Well done. When the gas permeability in the surface direction is smaller than 0.05 ml / min / Pa, the reaction gas cannot be sufficiently conveyed to the catalytic reaction layer, which is not preferable. On the other hand, when the gas permeability in the surface direction is larger than 0.10 ml / min / Pa, the flow of the reaction gas becomes strong and the drying of the polymer film is likely to proceed.

<Measurement method of gas permeability in surface direction>
As shown in FIG. 2, a porous carbon electrode base material cut out to 36 mmφ in a cylindrical compression jig (compression portion area 6.28 cm ² ) having an outer shape of a pressurizing portion of 30 mmφ and a diameter of a gas flow portion of 10 mmφ. A load 5 corresponding to 1 MPa is applied. The pressure difference between the inside of the substrate and the outside of the substrate when air 4 is allowed to flow from above the cylinder at a flow rate of 200 ml / min is calculated from the following equation. The gas permeation area is 0.785 cm ² .
Gas permeability (ml / min / Pa) = flow rate (ml / min) / pressure difference (Pa) between the inside and outside of the substrate.

<(Gas permeability in penetration direction) / (Gas permeability in plane direction)>
The porous carbon electrode substrate according to the present invention has a (gas permeability in the penetration direction) / (gas permeability in the plane direction) of 100 to 200. When (the gas permeability in the penetration direction) / (the gas permeability in the plane direction) is smaller than 100, that is, when the gas permeability in the plane direction is too high compared to the penetration direction, the reaction gas hardly flows in the penetration direction. , Because it is discharged before it sufficiently penetrates the catalyst layer. On the other hand, when (gas permeability in the penetration direction) / (gas permeability in the plane direction) is larger than 200, the reaction gas does not reach the entire surface of the catalyst layer in the plane direction. More preferably, it is 120-170.

<Measurement method of gas permeability in penetration direction>
The base when the porous carbon electrode substrate is sandwiched by the same method as the gas permeability measurement method in the plane direction, the gas flow path is changed to the penetration direction, and air is flowed from above the cylinder at a flow rate of 200 ml / min. The pressure difference between the upper part of the material and the lower part of the base material is measured and calculated from the following formula. The gas permeation area is 0.785 cm ² .
Gas permeability (ml / min / Pa) = flow rate (ml / min) / pressure difference (Pa) between the upper part of the substrate and the lower part of the substrate.

<Bulk density>
The porous carbon electrode substrate according to the present invention preferably has a bulk density of 0.23 to 0.30 g / m ² . More preferably, it is 0.24-0.29 g / m < ² >. When the bulk density is smaller than 0.23, the bonded short carbon fiber paper may be easily peeled off. Moreover, when the bulk density is greater than 0.30, gas permeability and drainage may be deteriorated.

<Measurement method of bulk density>
First, the thickness of the porous carbon electrode substrate is measured using a thickness measuring device (trade name “Dial Thickness Gauge 7321”, manufactured by Mitutoyo Corporation). In this case, the size of the probe is 10 mm in diameter, and the measurement pressure is 1.5 kPa. The bulk density is calculated by the following formula using the measured thickness. The basis weight can be measured with an electronic balance.
Bulk density (g / cm ³ ) = basis weight (g / m ² ) / thickness (mm) / 1000
<Maximum surface pressure>
The porous carbon electrode substrate according to the present invention preferably has a maximum surface pressure of 1.0 to 2.0 MPa when a pressure of 1.0 MPa is applied. More preferably, it is 1.0-1.8 MPa.

  The maximum surface pressure is a pressure value at a portion where the highest pressure is applied when a pressure distribution is generated when pressure is applied to the porous carbon electrode substrate. The pressure distribution can be measured by applying pressure by overlapping a pressure sensitive paper or the like with the porous carbon electrode substrate. An example is shown in FIG. In the case of a large pressure distribution, there are many dark portions, that is, high pressure portions, and the pressure value becomes large. The pressure distribution is generated due to thickness spots or weight spots of the porous carbon electrode base material, dispersion spots of short carbon fibers, and the like, and the pressure concentrates on the spots. When the maximum surface pressure is larger than 2.0 MPa, it is not preferable because the durability of the fuel cell may be lowered, for example, a large damage is caused to the electrolyte membrane when the surface pressure is high when the cells are assembled. In general, when the bulk density is high, the pressure distribution tends to be large even if there are few thickness spots.

<Measurement method of maximum surface pressure>
A sample of a porous carbon electrode substrate was cut into 20 mm × 20 mm, and a porous carbon electrode substrate and a pressure measurement film (trade name: “Prescale LLW”, Fuji on a universal compression tester (manufactured by Imada Manufacturing Co., Ltd.) Film Co., Ltd.) is placed and compressed at a pressure of 1 MPa. After compression, it was confirmed that the pressure measurement film was colored as shown in FIG. 3, and the pressure was read with a pressure analysis system (trade name: “FPD-9210”, manufactured by Fuji Film Co., Ltd.). The portion showing the highest pressure is defined as “maximum surface pressure”.

  Hereinafter, the present invention will be described more specifically with reference to examples, but the present invention is not limited thereto.

Example 1
Polyacrylonitrile (PAN) -based carbon fiber (7 μm diameter CF) having an average fiber diameter of 7 μm and an average fiber length of 3 mm (trade name: “Pyrofil TR50S”, manufactured by Mitsubishi Rayon Co., Ltd.) 86 parts by mass, polyvinyl alcohol (PVA) ( 14 parts by mass of a product name: “VPB107” (manufactured by Kuraray Co., Ltd.) was uniformly dispersed using 10,000 parts by mass of water as a dispersion medium. Thereafter, the paper was continuously made with a wet continuous paper machine. The moisture content of the stock after papermaking was 89%. Subsequently, the paper stock after papermaking was sandwiched between two papermaking felts (manufactured by Nippon Felt Co., Ltd.) without being pressed (pressing pressure: 0 MPa). The moisture content of the stock after paper making was sandwiched between the paper felts was 82%, as before. Then, it was made to contact with a hot roll and dried to obtain a long carbon short fiber paper having a carbon fiber basis weight of about 21 g / m ² and wound into a roll.

  This carbon short fiber paper was continuously coated with a 28% by mass methanol solution of a phenolic resin (trade name: “Phenolite J-325”, manufactured by DIC Corporation) from both sides by the kiss coating method. The carbon short fiber paper containing a phenol resin was obtained by drying for minutes, and it wound up in roll shape.

  Two carbon short fiber papers are laminated so that the bottom surface of the carbon short fiber paper containing the phenolic resin is on the surface, and then sandwiched with a release agent coating substrate, and continuously with a double belt press device. Were subjected to heat pressing (maximum load during pressing: 10 MPa) to obtain a resin-cured carbon short fiber paper. When the cross section of the resin-cured short carbon fiber paper was viewed, it was observed that a large amount of phenol resin cured product was unevenly distributed on the surface layer. A cross-sectional photograph of the resin-cured short carbon fiber paper by a fluorescence microscope is shown in FIG.

Subsequently, the resin-cured short carbon fiber paper is passed through a continuous firing furnace having a maximum temperature of 800 ° C. for 10 minutes in a nitrogen gas atmosphere, and then heated for 10 minutes in a continuous firing furnace having a maximum temperature of 2200 ° C. to be carbonized. Thus, a porous carbon electrode substrate having a length of 100 m was continuously obtained. Table 1 shows the physical properties of the porous carbon electrode substrate. The bulk density of the obtained porous carbon electrode substrate was 0.25 g / cm ³ . The bending strength ratio (MD / CD) in the MD direction and the CD direction is 2.0, which is oriented vertically, and the gas permeability in the plane direction and the gas permeability in the penetration direction / (the gas permeability in the plane direction). This value was also good, and it was suitable as an electrode substrate used for a gas diffusion layer for a fuel cell.

(Example 2)
A porous carbon electrode substrate was continuously obtained in the same manner as in Example 1 except that the pressing pressure continuously applied by the double belt press apparatus was 14 MPa. Table 1 shows the physical properties of the porous carbon electrode substrate.

(Example 3)
A porous carbon electrode substrate was continuously obtained in the same manner as in Example 1 except that the fiber orientation was changed from 2.0 to 4.0. Table 1 shows the physical properties of the porous carbon electrode substrate.

(Comparative Example 1)
The porous carbon electrode substrate was continuously formed in the same manner as in Example 1 except that the paper stock after paper making was sandwiched between felts for paper making and pressed at 0.2 MPa to make the moisture content 70% by mass. I got it. The porous carbon electrode base material has a large (gas permeability in the penetration direction) / (gas permeability in the plane direction) (that is, a gas permeability in the plane direction is smaller than that in the penetration direction). It was not suitable as an electrode substrate used for the diffusion layer.

(Comparative Example 2)
As in Example 1, two carbon short fiber papers containing phenolic resin were cut into 20 cm × 20 cm, overlapped so that the orientation directions of the carbon short fibers were orthogonal, sandwiched between release agent coating substrates, and in a batch apparatus Heat press for 3 minutes (maximum load during pressing: 1.3 MPa) to obtain a resin-cured carbon short fiber paper. Subsequently, the resin-cured carbon short fiber paper was fired for 10 hours in a continuous firing furnace having a maximum temperature of 2000 ° C. to obtain a porous carbon electrode substrate. MD / CD of the obtained porous carbon electrode base material was 1.1. Since the porous carbon electrode base material has a large (gas permeability in the penetration direction) / (gas permeability in the plane direction) as in Comparative Example 1, the porous carbon electrode base material is used as an electrode base material used for a gas diffusion layer for a fuel cell. Was not appropriate.

(Comparative Example 3)
A porous carbon electrode substrate was continuously obtained in the same manner as in Example 1 except that the pressing pressure continuously applied by a double belt press apparatus was 20 MPa. The porous carbon electrode base material has a high bulk density, and the gas permeability is lower in both the surface direction and the penetration direction than in the examples, and is not suitable as an electrode base material used for a gas diffusion layer for a fuel cell.

(Reference example)
A fiber bundle of polyacrylonitrile (PAN) -based carbon fibers having an average fiber diameter of 7 μm was cut to obtain short fibers having an average fiber length of 3 mm. Next, 100 parts by mass of the short fiber bundle and 200 parts by mass of vinylon fibers having a diameter of 100 μm are defibrated in water, and when sufficiently dispersed, the short fiber of polyvinyl alcohol (PVA) as a binder is 60 parts by mass. Then, paper was made using a standard square sheet machine according to JIS P-8222 method. The obtained carbon short fiber paper had a mass per unit area of 90 g / m ² .

  This carbon short fiber paper is immersed in a 10% by mass methanol solution of a phenol resin (trade name: “Phenolite J-325”, manufactured by DIC Corporation) and pulled up to 100 parts by mass of the carbon short fiber paper. 200 parts by mass were attached and dried with hot air. Thereafter, two carbon short fiber papers impregnated with the phenol resin were stacked and sandwiched between release papers, and placed in a batch press apparatus under conditions of 180 ° C. and 1.3 MPa for 5 minutes to cure the phenol resin.

  Then, the porous carbon electrode base material was obtained by heating at 2000 degreeC in a nitrogen gas atmosphere at 2000 degreeC for 1 hour, and carbonizing. The porous carbon electrode base material has higher gas permeability in the surface direction compared to the examples, and since the maximum surface pressure is high, there is a possibility that local pressure is applied to peripheral members when cells are assembled. From the viewpoint of durability, it was not appropriate.

1 Papermaking after papermaking 2 Felt for papermaking 3 Heat roll 4 Air 5 Load

Claims (5)

A method for producing a porous carbon electrode substrate in which two or more carbon short fiber papers are laminated via a phenolic resin carbide, and the degree of fiber orientation of the carbon short fibers is 2 to 5,
(1) A step of making a paper material containing short carbon fibers and a binder,
(2) The paper stock after paper making is continuously sandwiched between felts for paper making and pressed at a pressure of 0 to 0.05 MPa to make the moisture content 80 to 85%, and then dried to produce carbon short fiber paper. Obtaining a step,
(3) impregnating the carbon short fiber paper with a phenol resin, and laminating two or more carbon short fiber papers impregnated with the phenol resin;
(4) A step of carbonizing the laminated short carbon fiber paper while pressing the phenol resin by pressing at a pressure of 5 to 18 MPa while heating.
And a method comprising:   The method for producing a porous carbon electrode substrate according to claim 1, wherein the binder is polyvinyl alcohol.   The method for producing a porous carbon electrode substrate according to claim 1 or 2, wherein in the step (3), the two or more carbon short fiber papers are laminated so that a papermaking lower surface side is a surface.   The said process (3) WHEREIN: Impregnation of the phenol resin to the said carbon short fiber paper is what makes a phenol resin adhere to both surfaces of the said carbon short fiber paper by a kiss coat method. The manufacturing method of the porous carbon electrode base material of description.   The amount of the phenol resin impregnated in the carbon short fiber paper in the step (3) is 70 to 150 parts by mass with respect to 100 parts by mass of the carbon short fibers in the carbon short fiber paper. The manufacturing method of the porous carbon electrode base material of any one.